1. Field of the Invention
The present invention relates to the field of sensors or transducers for converting physical forces into an electrical signal and, more particularly, to a novel piezoelectric device of this type having the ability to cancel the capacitance of piezoelectric crystals used in the device so as to provide a signal gain with improvement in signal to noise ratio.
2. Brief Description of the Prior Art
Transducers of the self-generating, piezoelectric type have been employed in the past for measurement of dynamic force, pressure or acceleration. A specific type of such transducers or sensors employs an integral, unity gain, FET input, impedance converting amplifier such as is disclosed in U.S. Pat. No. 3,569,747. The piezo crystal element is usually made with two or more quartz, or other material, crystals, and when properly connected to the amplifier, generates a charge Q which, by virtue of capacitance C, forms a voltage V at the input of the amplifier. The following electrostatic equation defines the voltage and resultant signal as follows: EQU V=(Q/C) EQ 1
V=Voltage at gate of FET, Volts PA1 Q=Total charge generated by piezo element due to measurand input, picocoulombs PA1 C=Total shunt capacitance including the piezo element capacitance, the stray capacitance and the input capacitance of the FET, picofarads
The amplifier used in conventional circuits utilizes unity gain, non-inverting common drain amp (or source follower), and a voltage V will appear at a source terminal superimposed upon a quiescent DC voltage at the source terminal. The power unit consists of a DC voltage source such as a battery or a power supply and a constant current device to serve as a remote source load. A DC blocking capacitor in the power unit blocks the source voltage bias so as to return the dynamic signal to a zero voltage baseline.
Common drain voltage gain amplifiers have been employed to increase the signal level but problems and difficulties exist with such conventional gain circuits. By reducing the negative voltage feedback to the amplifier, these circuits can boost the output voltage measurably; however, because of the Miller effect which multiplies input capacitance by amplifier gain, the effective gain is reduced, in accordance with the electrostatic equation, EQ 1. This increase in input capacitance actually decreases the voltage across the piezoelectric crystal due to the Miller effect, reducing the effectiveness of voltage gain circuits. Further, these gain circuits amplify the FET broad band noise as well as the signal, resulting in no increase in resolution.
Therefore, a long-standing need has existed to employ a high-sensitivity, high-resolution piezoelectric sensor which utilizes electronic feedback to cancel the capacitance of the piezoelectric crystals thereby providing gain with attendant increase in signal to noise level.